1. Field of the Invention
This invention relates to an information processor for electrically recording/reproducing information based on an application of the principle of a scanning tunnel microscope (STM) or an atomic force microscope (AFM), and an information processing method using the information processor.
2. Description of the Related Art
Recently, with the development of an information-based society, many attempts have been made to develop large-capacity memories.
Such memories are variously designed by considering the desired performance and the intended use, and the following are general performance requirements for such memories:
(1) high recording density and large recording capacity;
(2) high recording/reproduction response speed;
(3) low power consumption; and
(4) high productivity and low price. Presently, the development of memory systems or memory media satisfying these requirements is eagerly being promoted.
In the past, magnetic memories formed of magnetic materials and semiconductor memories formed of semiconductors have mainly been provided. Recently, with the development of laser technology, low-priced high-density recording media have been realized with optical memories using organic pigment, photopolymers or the like.
To further increase the density and capacity of this kind of memory, technology for forming finer unit memory bits is being promoted. In addition, memories based on principles entirely different from those of the above-mentioned conventional memories have been proposed. For example, molecular electronic devices have been conceptualized in which organic molecules function as logical elements and memory elements. Such molecular electronic devices can be regarded as devices having the unit memory bit size reduced to an utmost limit. However, such devices encounter the problem of establishing a method of accessing each molecule.
On the other hand, scanning tunnel microscopes (STMs) [see, e.g., G. Binning et al., Phys. Rev. Lett., 49, 57 (1982)] have recently been developed, which enable a real space image to be measured with a high resolution irrespective of monocrystalline and amorphous states.
For STMs, a phenomenon is utilized in which a tunnel current flows between a metallic probe (electrode) and a conductive material if the distance therebetween is reduced to about 1 nm while a voltage is applied therebetween. The current caused in this manner is very sensitive to the change in the distance between the probe and the conductive material. Therefore, it is possible to read various kinds of information relating to a whole electron cloud in real space by scanning the probe so that the tunnel current is constantly maintained. In this case, the resolution in a direction normal to the surface is about 0.1 nm.
Accordingly, it is substantially possible to perform recording and reproduction at a high density on an atomic order (on the order of sub-nanometers) by applying the principle of STMs. For example, in the case of an information processor disclosed in Japanese Patent Laid-Open Publication No. 61-80536, writing is performed by removing atom particles attracted to a medium surface. The removing is performed using an electron beam or the like, and written data is reproduced using an STM. Also, a recording method, such as that disclosed in U.S. Pat. No. 4,575,822, has been proposed in which recording is performed by injecting a charge into a dielectric layer formed on a recording medium surface. The charge is injected using a tunnel current flowing between the medium surface and a probe electrode, or by physically or magnetically collapsing a medium surface using laser light, an electron beam, a corpuscular beam or the like.
A method has also been proposed in which a thin layer of a material having a memory effect with respect to a switching characteristic of a voltage or current, e.g., a .pi. electron organic compound or a chalcogen compound, is used with an STM to perform recording and reproduction (see, e.g., Japanese Patent Laid-Open Publication Nos. 63-161552 and 63-161553). According to this method, recording and reproduction of a large capacity, e.g., a density of 10.sup.12 bit/cm.sup.2 when the recorded bit size is 10 nm, can be performed.
Reproduction methods for the above-described information processor include, for example, a method of detecting differences in a level of a recording medium surface corresponding to recorded bits by scanning the recording medium surface with a probe electrode while maintaining a constant current flow between the probe electrode and the recording medium surface, and a method of utilizing a phenomenon that a current flowing between a probe electrode and a recording medium surface increases in a high-conductivity region corresponding to a recorded bit, i.e., the current is detected to reproduce data represented by recorded bits.
However, in a case when the former reproduction method is adopted, preexisting differences in the level of the medium surface and actual recorded bits in the medium surface cannot be discriminated from each other based solely on an amount of movement of the probe electrode, because the probe can follow even small differences in level in the recording medium surface. Also, in this case, high-speed scanning is difficult because a maximum controllable scanning frequency is determined by an upper limit of the range of a feedback control circuit for maintaining a constant current.
In a case when the latter reproduction method is adopted, the current also is changed by differences in level in the medium surface. These changes in the current caused by differences in the level of the medium surface cannot be discriminated from changes due to recorded bits, as in the case of the former method, although high-speed scanning is possible.
Another problem may be encountered as described below. The probe electrode and the medium surface are maintained at a certain distance from each other, and this spacing serves as an insulating barrier. This barrier, however, is common to recorded bit writing areas and non-writing areas, and acts effectively as a tunnel resistance inserted in series. Therefore, if the distance between the probe electrode and the medium surface is changed, the ratio of current detected at writing areas to current detected at non-writing areas may become so large (or small) that recorded bits cannot be read accurately.
Accordingly, it is necessary to form a reproduction signal so that components caused by differences in the level of the recording medium surface are removed. It is also necessary to minimize and maintain at a constant level the tunnel resistance of the insulating barrier between the probe electrode and the recording medium surface so that the reproduction signal ratio based on the existence/non-existence of recorded bits is maximized. It is also preferable that the change in the applied voltage for recording cannot easily influence the control of the distance between the probe electrode and the medium.
Further, if the distance between the recording medium and the probe electrode is large, the resolution of the STM is reduced. That is, in terms of recording density, it is also preferable to minimize the distance between the recording medium and the probe electrode.
To remove components caused by differences in the level of the medium surface from the reproduction signal, and to remove the effects of the applied voltage on the distance control at the time of recording, a method of maintaining a constant distance between the medium surface and the probe using a quantity other than the current flowing therebetween is needed. An atomic force acting between the medium surface and the probe is an example of such a quantity. An atomic force microscope (AFM) for controlling the distance using the atomic force acting between the medium surface and the probe is disclosed in Japanese Patent Laid-Open Publication No. 1-245445.
In an AFM, a probe electrode is supported by an elastic body, a force acting between a probe electrode tip and a recording medium surface is balanced with a spring force of a deformation of the elastic body, and feedback control is performed so that the amount of this deformation is constant.
For this AFM control, a distance control is based on an atomic force acting between the medium surface and the probe electrode tip. To realize an atomic resolution, therefore, a probe electrode having a very sharp extreme end (a tip curvature ordinarily of 0.1 .mu.m or less) is used. If such a probe electrode is used, the above-mentioned atomic force acts on a very small area (on an atomic order) such that the density of the force applied to the medium surface is very high. For this reason, if organic molecules are used as the medium, there is a possibility of a deformation of the medium surface or a separation of the medium caused by a local force.
Further, the resistance to the movement of the probe electrode during scanning is increased by the interatomic force so that the stability of scanning is reduced. It is therefore difficult to stably read changes in electrical characteristics and the like of the recording medium surface.